Recommended dosage is 1000 mg fish oil (containing 300 mg combined EPA/DHA) per 30 pounds (14 kg) of body weight. Maximum dosage for dogs with health problems would be 1000 mg fish oil (300 mg EPA/DHA) per 10 pounds (4.5 kg) of body weight. You can also use sardines in place of fish oil supplements; one small sardine supplies over 100 mg EPA/DHA.
Vitamin E should also be given whenever oils are supplemented (even small amounts are adequate, but highest recommended dosage would be 100 IU per day for small dogs, 200 IU for medium-sized dogs, and 400 IU for large dogs). Note that fish oil is not the same as cod liver oil, which is high in vitamins A and D.